This invention relates generally to the recovery of marketable hydrocarbons such as oil and gas from hydrocarbon bearing deposits such as heavy oil deposits or tar sands by the application of electrical energy to heat the deposits. More specifically, the invention relates to the heating of such deposits from a single borehole and recovering hydrocarbons from such borehole wherein the deposits are heated by the controlled application of electrical power at the deposit. Still more specifically, the invention relates to the controlled and efficient application of power and withdrawal of liquid hydrocarbons to vaporize water in the upper portion of a deposit and maintain an annular region of water vapor extending from the borehole into the upper portion of deposit, thereby providing a non-conductive dielectric for directing electrical power deeper into the deposit.
In many deposits, especially in medium and heavy oil deposits, the viscosity of the oil impedes flow, especially in the immediate vicinity of the borehole through which the oil is being produced. As all of the oil must flow into the borehole, the mobility of the fluid in the immediate vicinity of the borehole dominates the production rate, wherefore any impediment to fluid flow at the borehole is particularly unwelcome. It has, therefore, been known to heat the formations, particularly in the vicinity of the borehole, to lower the viscosity of the liquids in the deposit and, hence, provide greater mobility and more profitable production.
Steam injection has been used to heat the deposit to reduce the viscosity of oil in the immediate vicinity of a borehole, and to some extent steam can be used as a heat transport medium. Steam injection can be used in some deposits for economically stimulating production. However, if steam is injected from the surface, it loses a large amount of heat as it progresses down the hole, wastefully heating formations above the formations of interest. This has given impetus to the development of downhole steam generators, which have problems of their own. Further, the use of steam stimulation is uneconomic in many deposits.
As a consequence, a number of electrical heating methods have been considered. It is known to provide uniform heating of a deposit by interwell energization, as shown, for example, in Bridges and Taflove U.S. Pat. No. Re. 30,738. Such methods, however, require a relatively extensive array of boreholes and comprehensive development of a field, which is not always warranted. Single well heating is shown in Sarapuu U.S. Pat. No. 3,211,220, which shows the application of electrical power between an electrode in a formation and a distributed electrode at or near the earth's surface.
It has been recognized that single well stimulation is more effective if heat can be applied some distance into the formations from a borehole, as by causing electrical energy to flow into the formations some distance from the borehole. To this end, it has been suggested to extend the borehole laterally and extend the electrodes themselves out into the formations. See, for example, Kern U.S. Pat. No. 3,874,450, Todd U.S. Pat. No. 4,084,639, Gill U.S. Pat. No. 3,547,193, Crowson U.S. Pat. No. 3,620,300 and Orkiszewski el al. U.S. Pat. No. 3,149,672. All of such systems require special downhole development, generally requiring special tools or operations to clear out a portion of the formation for entry of the electrode.
In Crowson U.S. Pat. No. 3,620,300 is shown a method and system wherein not only the electrodes but insulating barriers are extended out into the formations, thereby increasing the effective diameter of the borehole. Such method and system require physical enlargement of the borehole to admit the enlarged electrodes and insulating barriers. Such method and system include the emplacement of such a barrier extending into the formation from the borehole above a single electrode (monopole) also extending into the formation from the borehole, as well as the emplacement of such barrier between a pair of vertically spaced electrodes (dipole) in the same borehole.